Seismic Anisotropy Across a Boundary Between Compression and Extension Above a Subducting Plate, Western North Island, New Zealand.
The North Island of New Zealand lies on the Australian Plate, above the subducted Pacific Plate. The highest point on the North Island is near its centre on Mt. Ruapehu Volcano, which is the southernmost in a line of active volcanoes associated with extension in the Taupo Volcanic Zone, the southern limit of the Lau-Havre Trough. South of Mt. Ruapehu, compression occurs and volcanism stops, but subduction continues. An east-west trending boundary between Mt. Ruapehu and Mt. Taranaki volcanoes is delineated by a strong gravity gradient, change in crustal thickness, and abrupt changes in seismic attenuation, and has been termed the "Taranaki- Ruapehu Line" (Salmon, 2007). Shear wave splitting determined from SKS phases on broadband stations exhibit strong, trench-parallel (extension-perpendicular) anisotropy both south of the line and north east of the line near the center of the extending region. However, SKS waves recorded north of the line and west of the extending region do not split, suggesting isotropy. (Greve and Savage, 2007). Anisotropy measurements from local S phases recorded on sparsely-spaced stations also suggest changes from well-aligned shear wave polarisations south of the line to more scattered measurements north of the line (Audoine et al., 2004). Several models have been proposed to explain the north-south changes. These include: 1) in the north, extension carries fluids away from the plate, allowing the fluids to spread westward while to the south, compression and plate- boundary parallel flow confine the fluids to the slab itself. (Audoine et al. 2004) 2) Thickened crust to the south shut off the vertical flow of fluids, driving northeastward flow of the mantle, increasing the fluids available in the north (Reyners et al. 2006). 3) In the west and central NI Miocene shortening of ~ 100 km led to a series of a Rayleigh-Taylor instabilities that have since detached to be replaced by an asthenospheric upwelling (Stern et al. 2006). We use seismic data gathered by seven temporary three-component stations set up across the Taranaki- Ruapehu line (TRL) to determine anisotropy in local S phases in an attempt to delineate more accurately the location of the change in anisotropy, and to explain the mechanism for the change. Preliminary results of shear- wave splitting delay times (dt) and fast polarisations are made from 409 phases. No overall correlation between dt and depth is found, which is attributed to a complex and rapidly changing anisotropic structure. Yet the anisotropy is consistent for phases arriving within close incidence angles and back azimuths; we attribute this to lateral variations in anisotropy. North of the line, within the extending region, extension-perpendicular fast orientations dominate with delay times from 0.5 to 0.8s, consistent with large splitting observed on SKS phases. Closer to the TRL, E-W trending fast orientations are more frequent and small delay times (0.1 to 0.3s) are common. We propose that the smaller delay times are caused by re-splitting in the crust due to anisotropic crustal structures parallel to the TRL. We plan to incorporate more measurements by using a newly developed automatic technique to test if the preliminary results hold.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 7203 Body waves;
- 7218 Lithosphere (1236);
- 7240 Subduction zones (1207;
- 8109 Continental tectonics: extensional (0905);
- 8120 Dynamics of lithosphere and mantle: general (1213)